The core idea: one number, and it is not the temperature
Netchūshō (熱中症) is literally illness from being struck by heat: netsu, heat, chū, to be hit, as in chūdoku, poisoning, and shō, illness. It covers everything from a dizzy spell to an emergency.
The number Japan uses to warn about it is the atsusa shisū (暑さ指数), literally hotness index, known internationally as WBGT. The Environment Ministry calculates it from three readings: 70 percent from a wet-bulb thermometer, which reflects humidity, 20 percent from a black globe that soaks up sun and heat radiating from pavements, and only 10 percent from ordinary air temperature. Humidity dominates because sweat that cannot evaporate does not cool you. The ministry even drops the degree sign, so 31 on the index is not 31 degrees. A presenter opens the forecast with the first two lines below; colleagues grumble with the last two.
| Japanese | Romaji | Meaning |
|---|---|---|
| 今日は暑さ指数が高くなる見込みです。 | Kyō wa atsusa shisū ga takaku naru mikomi desu. | The heat index is expected to be high today. |
| 熱中症に注意してください。 | Netchūshō ni chūi shite kudasai. | Please be careful of heat illness. |
| 今日も蒸し暑いですね。 | Kyō mo mushiatsui desu ne. | Humid and hot again today, isn't it? |
| 気温はそれほどでもないのに、湿度が高いですね。 | Kion wa sorehodo demo nai noni, shitsudo ga takai desu ne. | The temperature isn't that high, but the humidity is. |
The five bands of the heat index
On the ministry's live map, every point is coloured by band, and the band names are ordinary warning words. The diagram also shows the short instruction the Japan Sport Association's guideline attaches to each band for exercise; phrases such as sekkyokuteki ni suibun hokyū turn up on TV too. Above the bands sit the two alert thresholds, both inside kiken. The last line below is what a student in August tells a parent.
| Japanese | Romaji | Meaning |
|---|---|---|
| 今日は厳重警戒のレベルです。 | Kyō wa genjū keikai no reberu desu. | Today is at the strict vigilance level. |
| 暑さ指数が31以上だと、危険のレベルです。 | Atsusa shisū ga sanjūichi ijō da to, kiken no reberu desu. | At 31 or more on the heat index, it is the danger level. |
| 暑さ指数が高いので、部活は中止になりました。 | Atsusa shisū ga takai node, bukatsu wa chūshi ni narimashita. | Club practice was cancelled because the heat index is high. |
| Band | Reading | Index | Literal sense |
|---|---|---|---|
| 危険 | kiken | 31 or more | Danger |
| 厳重警戒 | genjū keikai | 28 to under 31 | Strict vigilance |
| 警戒 | keikai | 25 to under 28 | Vigilance, literally warn and guard |
| 注意 | chūi | 21 to under 25 | Care, attention |
| ほぼ安全 | hobo anzen | Under 21 | Almost safe |
The ministry notes that heat illness cases rise sharply once the index passes 28, the start of genjū keikai. The daily-life guideline it cites, from the Japanese Society of Biometeorology, uses only four bands and calls everything under 25 chūi, so you may see both versions.
The two alerts: keikai arāto and tokubetsu keikai arāto
The official names are netchūshō keikai jōhō and netchūshō tokubetsu keikai jōhō, heat illness alert information and special alert information, but everyone uses the nicknames with arāto, alert. Since April 2024, when the revised Climate Change Adaptation Act took full effect, the ordinary alert has had a legal basis and the special alert has existed alongside it. Friends mention it casually; the news uses the formal verb happyō saremashita, has been announced.
The two differ in what must happen and where. The ordinary alert needs just one observation point in a forecast area to be forecast at 33. The special alert needs every point in a whole prefecture, apart from a few listed exceptions, to be forecast at 35: severe, widespread heat. The diagram sets them side by side.
| Japanese | Romaji | Meaning |
|---|---|---|
| あしたは熱中症警戒アラートが出ているよ。 | Ashita wa netchūshō keikai arāto ga dete iru yo. | There's a heatstroke alert out for tomorrow. |
| 県全体に熱中症特別警戒アラートが発表されました。 | Ken zentai ni netchūshō tokubetsu keikai arāto ga happyō saremashita. | A special heatstroke alert has been issued for the whole prefecture. |
| 市内のクーリングシェルターが開放されます。 | Shinai no kūringu sherutā ga kaihō saremasu. | The city's cooling shelters will be opened. |
| Feature | Keikai arāto | Tokubetsu keikai arāto |
|---|---|---|
| Trigger | Any one point forecast at 33 or more | Every point in the prefecture forecast at 35 or more |
| Area | One of 58 forecast areas | A whole prefecture |
| Announced | About 5 pm the day before and 5 am on the day | About 2 pm the day before |
| What else happens | Ministry and broadcasters urge prevention | Designated cooling shelters open |
What the TV and the loudspeaker actually say
When an alert is out, the same sentences come from the ministry, the evening news and the town loudspeaker, as polite requests with kudasai or the let's form mashō. Suibun hokyū is literally water-part replenishment, and komame ni means little and often. Eakon is the everyday word for the air conditioner; kūrā, cooler, is older.
Tekisetsu ni, appropriately, carries a quiet message. Many older people switch the air conditioner off to save money or because they feel the heat less, so in practice the phrase means do not hold back.
| Japanese | Romaji | Meaning |
|---|---|---|
| 熱中症警戒アラートが発表されています。 | Netchūshō keikai arāto ga happyō sarete imasu. | A heatstroke alert has been issued. |
| エアコンを適切に使用してください。 | Eakon o tekisetsu ni shiyō shite kudasai. | Please use your air conditioner appropriately. |
| エアコンを我慢せずに使いましょう。 | Eakon o gaman sezu ni tsukaimashō. | Use the air conditioner rather than putting up with the heat. |
| こまめに水分補給をしてください。 | Komame ni suibun hokyū o shite kudasai. | Drink fluids little and often. |
| のどが渇く前に水を飲みましょう。 | Nodo ga kawaku mae ni mizu o nomimashō. | Drink water before you get thirsty. |
| 外出はなるべく避けてください。 | Gaishutsu wa narubeku sakete kudasai. | Please avoid going out as far as possible. |
| 命に関わる危険な暑さです。 | Inochi ni kakawaru kiken na atsusa desu. | This is dangerous, life-threatening heat. |
Beyond the basics: cooling shelters and looking out for others
The 2024 law also lets mayors designate shitei shonetsu hinan shisetsu, literally designated heat-refuge facilities, better known as kūringu sherutā, cooling shelters. They are places with air conditioning, such as community centres, libraries and shopping centres, and they open at set hours when a special alert covers the area. Your city's website lists them; some towns also run places with names like suzumi-dokoro, a place to cool off, which may follow their own rules.
The ministry's messages also ask people to watch over others. Mimamori (見守り) is literally watching and protecting, and koekake (声かけ) is literally hanging your voice on someone: calling out to check they are all right. In this scene, a younger resident checks on the elderly man next door, asks the city office about shelters, then comes back with an invitation.
| Japanese | Romaji | Meaning |
|---|---|---|
| 暑いですね。エアコン、つけていますか。 | Atsui desu ne. Eakon, tsukete imasu ka. | Hot, isn't it? Do you have the air conditioner on? |
| 何かあったら、声をかけてくださいね。 | Nanika attara, koe o kakete kudasai ne. | If anything happens, just give me a shout. |
| この近くにクーリングシェルターはありますか。 | Kono chikaku ni kūringu sherutā wa arimasu ka. | Is there a cooling shelter near here? |
| 図書館で少し涼んでいきませんか。 | Toshokan de sukoshi suzunde ikimasen ka. | Shall we cool off in the library for a while? |
Symptoms: the words for feeling the heat
The ministry's manual groups heat illness symptoms into three degrees, read ichi-do, ni-do and san-do, from mild to severe, and says the key question is whether the person is fully conscious. The vocabulary below is what you need to describe symptoms to staff, colleagues or an emergency operator; the decisions belong to the manual, the ministry's first-aid page and, in an emergency, 119. General illness words have their own post. Here, a visitor at a summer festival speaks to a steward; the table then lists the words behind such sentences.
| Japanese | Romaji | Meaning |
|---|---|---|
| すみません、気分が悪いです。 | Sumimasen, kibun ga warui desu. | Excuse me, I feel unwell. |
| 頭がくらくらします。 | Atama ga kurakura shimasu. | My head is spinning. |
| 足がつりました。 | Ashi ga tsurimashita. | I've got a cramp in my leg. |
| 吐き気がします。 | Hakike ga shimasu. | I feel sick. |
| 救急車を呼んでください。 | Kyūkyūsha o yonde kudasai. | Please call an ambulance. |
| Word | Reading | Meaning |
|---|---|---|
| めまい | memai | Dizziness |
| 立ちくらみ | tachikurami | Light-headedness on standing, literally standing darkness |
| こむら返り | komuragaeri | Muscle cramp, usually in the calf |
| 大量の汗 | tairyō no ase | Heavy sweating |
| 頭痛 | zutsū | Headache |
| 吐き気 | hakike | Nausea |
| だるい | darui | Heavy, sluggish, drained |
| 意識がもうろうとする | ishiki ga mōrō to suru | Drifting in and out, confused |
Confusable pairs: atsui, netsu and keihō
Heat vocabulary is full of near-twins. The most useful distinction is the last one in the table: a heat alert is information, jōhō, not one of JMA's formal weather warnings, keihō, such as ōame keihō for heavy rain, even though the word keikai is inside its name.
| Pair | Reading | The difference |
|---|---|---|
| 暑い / 熱い | atsui / atsui | Hot weather versus hot to the touch; the heat index uses 暑 |
| 熱 / 熱中症 | netsu / netchūshō | A fever, or heat in general, versus heat illness |
| 日射病 / 熱中症 | nisshabyō / netchūshō | Old word for sunstroke, literally sun-ray illness, versus the wider term, which includes indoor cases |
| 気温 / 暑さ指数 | kion / atsusa shisū | Air temperature versus the heat index |
| 警戒 / 警報 | keikai / keihō | Vigilance, the word inside the alert, versus JMA's formal weather warnings |
Common mistakes with heat vocabulary
Reading the index as a temperature. A newcomer hears 33 and thinks that sounds like an ordinary summer day. The cause is that the index looks like degrees. The fix is to remember it runs lower: the sports guideline pairs an index of 31 with an air temperature of roughly 35 degrees.
Saying netsu ga arimasu to mean heatstroke. Netsu ga aru means I have a fever. The cause is that netsu also means heat. Say netchūshō kamo shiremasen, it might be heat illness, or describe the symptom.
Treating the alert as an outdoor, daytime matter. The cause is the old word nisshabyō, sunstroke, which puts the sun at the centre. The ministry's first message is about indoors, using the air conditioner, and heat illness can occur at night.
Thinking the special alert is just a stronger ordinary alert for the same area. It has a different trigger, every point rather than one, and covers a whole prefecture. When you hear tokubetsu, expect the cooling shelter announcements to follow.